The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to the compensation of fast spin echo pulse sequences to reduce image artifacts.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this xe2x80x9cMRxe2x80x9d signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. Most MR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts.
There are pulse sequences which enable scans to be completed in seconds rather than minutes. One of these is the Rapid Acquisition Relaxation Enhanced (RARE) sequence which is described by J. Hennig et al in an article in Magnetic Resonance in Medicine 3,823-833 (1986) entitled xe2x80x9cRARE Imaging: A Fast Imaging Method for Clinical MR.xe2x80x9d The RARE sequence is a fast spin echo sequence which utilizes RF refocused echoes generated from a Carr-Purcell-Meiboom-Gill sequence. Such fast spin echo (xe2x80x9cFSExe2x80x9d) scans are very susceptible to image artifacts caused by such things as eddy currents, B0 instability, gradient amplifier infidelity, magnetic hysteresis and high order Maxwell terms. In U.S. Pat. No. 5,378,985 issued to R. S. Hinks on Jan. 3, 1995 and entitled xe2x80x9cFast Spin Echo Prescan For MRI Systemsxe2x80x9d, a method is disclosed for measuring some of the phase errors prior to each patient scan and altering the fast spin echo pulse sequence to compensate for these errors. While this method provides a substantial reduction in ghost image artifacts, in certain situations where multiple root causes of phase error coexist and interact with each other, artifact free FSE images are difficult to produce.
The present invention is a process that is performed prior to an FSE scan to adjust the FSE pulse sequence and to thereby reduce phase errors produced by the MRI scanner. More specifically, the process includes: performing the FSE pulse sequence to acquire echo signals from the center of k-space, calculating from the acquired echoes a phase shift and a readout gradient compensation pulse to compensate the FSE pulse sequence for first order phase shifts along the readout gradient axis, performing the FSE pulse sequence to acquire additional echo signals from the center of k-space, calculating from the additional echoes a phase shift and a phase-encoding gradient compensation pulse to compensate the FSE pulse-sequence for zeroth and first order phase shifts along the phase-encoding gradient axis, performing the FSE pulse sequence to acquire another echo signal from the center of k-space, and calculating from the another echo signal a slice-select gradient compensation pulse to compensate the FSE pulse sequence for phase shifts along the slice-select gradient axis. As each gradient axis is compensated, the FSE pulse sequence used to acquire additional central k-space echoes is modified to compensate phase errors.